Because viruses lurk inside host (e.g., human) cells where antibodies cannot reach them, the only way they can be eliminated is by killing the infected host cell. To do this, the immune system uses a kind of white blood cell, called killer T cells. These cells act only when they encounter another cell that carries a "marker" (i.e., a protein) that links it to a foreign protein-- that of the invading virus. Killer T cells can themselves become infected by HIV or other viruses, or transformed by cancer. Also known as cytotoxic T cells (or cytotoxic T lymphocytes). See NK (natural killer) Cells; Null Cell; T Cells.